The long-term objective of our work is to produce a comprehensive set of vaccines that would effectively protect a large population against even genetically-engineered pathogens. The immediate objective of the proposal described here is to screen the whole genome of Yersinia pestis for the most potent vaccine candidates. Plague is a significant target for at least two reasons. 1) It is an extremely virulent, often lethal disease that has been suffered by humans since antiquity. 2) There is an immediate need to produce a broadly effective, safe, and inexpensive vaccine to counter the threat of bioterrorism. The immunogenic simplicity of a subunit vaccine currently under study may limit its ability to protect the diverse human population, and may render it ineffectual against a genetically-modified pathogen. Our phase I specific aims are to i) establish the informatics required to support a technically demanding, functional screen of the entire Y. pestis genome for protective antigens, ii) identify protocols for producing a complete Y. pestis gene library iii) define the optimal gene dose and design for this genome screen by testing Y. pestis genes in immune and protection assays. Our experimental approach combines three of our technologies: genetic immunization, expression library screening, and linear gene expression elements. In phase II we will apply this system to evaluate every Y. pestis gene as a PCR product in an arrayed matrix for protection of mice against infection. Positively scoring genes will constitute a portfolio of antigenic options to protect a broad population against natural and unnatural Y. pestis infections. In addition to identifying the components for a future vaccine, these protective antigens will reveal immune markers of protection, and thereby facilitate clinical validation of these as well as other potential plague vaccines.